This invention relates generally to the field of mixing or compounding reinforcing fiber strands with thermoplastic resin.
The fiber strands may be of any predetermined length and are introduced into an admixture with thermoplastic resins for immediate use in molding machines at a product molding facility.
Processes are known for combining reinforcing fiber strands with thermoplastic resin to form reinforced plastic composites. U.S. Pat. Nos. 4,439,387 and Re. 32,772 sharing common inventorship herewith disclose the embedding of thermoplastic resin in reinforcing fiber strands as they are drawn through a forming die having a convoluted passage, in the presence of molten thermoplastic resin introduced from an extruder. The extrusion product of that process comprises an elongated bar or rod having a continuous length of reinforcing fiber encased within thermoplastic resin. That preformed composite may be inserted into a die of an injection molding machine, and utilized as an insert in a compound, composite product comprising an additional layer of thermoplastic resin molded over the insert. It is also known to cut the extrudate rod from the aforesaid forming die into short lengths for use as molding pellets. In the process of the aforesaid United States patents, the extruded rod comprising a fiber/resin composite is immediately cooled, prior to final forming and cutting to desired lengths.
U.S. Pat. No. 5,185,117, also having identity of inventorship herewith, discloses a process for compounding thermoplastic resin and fiber strands in an extruding compounder. According to the process of that patent, melted thermoplastic resin is introduced into the compounding extruder along with reinforcing fiber strands. The resulting extrudate consists of a molten mass of thermoplastic resin having discrete lengths of fiber strands randomly dispersed therein. This hot mixture may then be fed directly into a preform device to produce a measured preform for use in a compression molding machine. In the disclosed process of the ""117 patent, the fiber strands are precut to desired lengths, before being introduced into the extruding compounder. The process and apparatus further requires a loss-in-weight scale to accurately measure predetermined quantities by weight of reinforcing fiber strands to be controllably introduced into the compounding extruder in the presence of thermoplastic resin. A separate loss-in-weight feed scale assembly is required to accurately convey predetermined amounts by weight of the thermoplastic resin into the compounding extruder for mixing in the desired proportions with the reinforcing fiber strands. The loss-in-weight scales necessarily add to the cost and complexity of the compounding apparatus.
U.S. Pat. No. 4,616,989 discloses an apparatus for incorporating glass fiber strands into thermoplastic resins in which a premixing chamber is utilized to initially mix glass fiber strands with molten resin. This mixture is then fed into a two-stage screw-type extruder to complete the mixing of the fiber strands and resin. The resin-fiber mass as discharged from the final extruder is passed through a forming die having a plurality of orifices. This serves to form the material into elongated filaments of glass fiber reinforced resin which are then cooled, and then granulated for use as a molding compound.
U.S. Pat. No. 2,877,501 to Brandt discloses a process for forming granules comprised of glass fiber strands coated with a molding plastic, which are intended for use as feed stock for an injection molding machine. In the Brandt process, fiber strands are pulled through an orifice within which they are coated with a resin material.
None of the known prior art processes for formulating a mixture of fiber reinforced resin for molding purposes are operatively effective for preparing a molding material comprised of fiber reinforced resin for immediate introduction into a molding machine at the same site where the fiber-resin mixture is made. Nor can any of the known prior art processes for formulating a mixture of fiber reinforced resin for molding purposes operate in a controlled and accurate discontinuous manner. Either cumbersome and costly apparatus, including multiple stage mixing devices and loss-in-weight scales, are required in the prior processes, and/or the fiber-reinforced resin is extruded into lengths, cut and packaged for sale and transportation to separate molding facilities.
There thus exists a need for a compact, efficient apparatus and process for accurately formulating mixtures of fiber and resin and thereafter directly introducing that molding material directly into a molding machine, such as an injection molding machine, a compression molding press, a transfer mold, a blow mold, a profile extrusion machine or an inject compression molding machine. In addition, there also exists a need for an apparatus and process capable of operating in a discontinuous manner to allow the admixture of variously sized batches of fiber reinforced resin molding material.
This invention has as its primary objective the provision of a process and apparatus for interspersing fiber strands in a thermoplastic resin in a desired weight ratio, under a controlled, fiber-coating process, and thereafter introducing the coated fiber strands directly into a molding machine at the same site, without cooling of the fiber-resin mixture. Another objective lies in the provision of a process and apparatus as described which may be operated in a discontinuous manner so as to produce discrete and varying amounts of the fiber-resin mixture.
These basic objectives are realized by threading at least one fiber strand through a coating die passage having an outlet and introducing into that passage a thermoplastic resin in a molten, pressurized state so that the thermoplastic resin flows from the passage through the outlet, thereby entraining the fiber strand in the flow of thermoplastic resin and coating the fiber strand with thermoplastic resin.
The coating die passage comprises at least one orifice of predetermined size that is larger in cross-sectional area than the cross-sectional area of the fiber strand. The remaining annular space between the fiber strand and the orifice through which molten resin passes defines a predetermined area through which the thermoplastic resin may flow. The ratio of the cross-sectional area of the fiber to the cross-sectional area of the annular space being the means whereby the proportion of resin coating to fiber strand may be controlled.
The process for interspersing fiber strands in a thermoplastic resin comprises the steps of conveying at least one fiber strand through a coating die in the presence of molten thermoplastic resin, thereby coating the fiber strand with the resin; and thereafter directly introducing the resin-coated fiber strand in a heated state into a receiver for movement into a molding machine. The receiver may comprise an extruder barrel which houses a rotatable screw or simply a plate or tray utilized to convey a pliant mass of resin and fiber into the mold press of a compression molding machine.
This process may be run in a discontinuous manner to produce a desired quantity of the molding material. The thermoplastic resin is typically introduced into the passage at pressures of between 4,000 psi and 40,000 psi. By way of example, a coating die operating at pressures between 12,000 and 14,000 psi resulted in the resin coated fiber strand becoming entrained with the resin flowing through the coating die at velocities of between 80 and 250 feet per minute. The resulting fiber-resin mixture comprises between 20 to 60 weight percent fiber strands.
The apparatus for wetting and conveying fiber strands with a thermoplastic resin of the present invention comprises a housing having a passage with a fiber inlet and an outlet. This passage is arranged and constructed to permit a continuous strand of said fiber to be passed from the inlet through the housing and out of the outlet. A resin injection port is fluidically connected to the passage to direct the thermoplastic resin into the passage under pressure. The pressurized flow of the thermoplastic resin into the passage contacts the continuous strand of fiber disposed through the passage and causes the continuous strand of fiber to be drawn through the housing. As the continuous strand of fiber is drawn through the housing, it is wetted by the thermoplastic resin.
The apparatus for coating fiber strands with a thermoplastic resin preferably comprises a resin-receiving chamber that is in fluidic communication with the resin injection port. In addition, the inlet to the passage through the housing may comprise a nozzle that extends into the passage. And, in order to prevent the backflow of resin into this nozzle, it may be desirable to form at least one back flow passage fluidically that is connected to the inlet to the passage of the housing.
In order to precisely adjust the fiber to resin ratio of the fiber reinforced resin molding material, a traction block having a bore formed therethrough may be connected directly to the coating die. The bore of the traction block is fluidically connected to the outlet of the passage through the housing of the coating die so as to allow the continuous strand of fiber and thermoplastic resin to pass therethrough. The diameter of the bore may be sized to pass therethrough the continuous strand of fiber and the thermoplastic resin in a predetermined weight ratio.
In order to optimize the flow of resin and fiber through the apparatus of the present invention, the geometry of the coating die may be arranged in a number of ways. For example, the passage and the injection port may be arranged such that their axes of symmetry intersect perpendicularly or at an acute angle. In addition, it may be desirable for the axis of symmetry of the injection port to intersect an annular chamber of the passage tangentially.
In an other embodiment of the present invention the apparatus for coating and conveying fiber strands with a thermoplastic resin comprises a housing having a fiber inlet and outlet with a fiber entrainment and resin coating passage within the housing connected between the fiber inlet and outlet for the passage of a continuous strand of fiber through the housing. A resin injection port in the housing is in fluid flow communication with the fiber entrainment passage to introduce pressurized resin in a fluid state into contact with the continues strand of fiber in the entrainment passage. A supply of continuous fiber strand is connected to the fiber inlet of the housing and a supply source of molten thermoplastic resin is placed in fluidic communication with the resin injection port in the housing. The pressurized flow of resin into the entrainment passage conveys the continuous strand of fiber through the housing and out of the fiber outlet while coating the fiber with the resin. The resin injection port may be arranged to open upon a resin receiving chamber within the housing having at least one flow passage extending from the resin receiving chamber to the fiber entrainment passage. The resin receiving chamber and the flow passage place the resin injection port in fluidic communication with the fiber entrainment passage. The flow passage extends in the general direction of extent of the fiber entrainment passage and thereby directs pressurized resin in the same general direction as that of fiber movement within the fiber entrainment passage, enhancing the movement of the fiber strand towards the fiber outlet.
A receiving device is constructed and arranged to receive the resin coated fiber from the outlet of the housing and deliver the resin coated fiber to a molding machine or the like positioned in close proximity thereto.
A fiber inlet nozzle having an outlet orifice that is in fluid flow communication with the fiber entrainment passage may be positioned within the fiber inlet in the housing. The outlet orifice of the nozzle is of a predetermined size to allow a fiber strand of predetermined diameter to pass therethrough while substantially preventing the backflow of pressurized resin from the fiber entrainment passage through the fiber inlet nozzle. In one embodiment, the nozzle has a tapered outlet end constructed and arranged to define at least one flow passage in fluid flow communication between the resin injection port and the fiber entrainment passage. More specifically, the at least one flow passage is defined between the tapered outlet of the nozzle and an adjacent wall of the housing. Alternatively, the nozzle may have a resin receiving chamber formed therein from which the at least one flow passage extends to the fiber entrainment passage, the chamber being in fluidic communication with the resin injection port within the housing. The at least one flow passage typically has its discharge end opening into the fiber entrainment passage in close proximity to the outlet orifice of the nozzle. In order to promote more laminar flow of the resin, the resin receiving chamber is preferably of an annular shape.
One or more back flow chambers may be located upstream from where the injection port communicates with the passage to prevent resin from escaping the coating die through the passage or nozzle.